Chapter 2. Simulation of Polymeric Membrane Systems for CO2 Capture

Abstract

Carbon Capture & Sequestration (CCS) is a key issue in the reduction of greenhouse gases emissions. The capture step, which corresponds to the most expensive part of the technological chain, can be potentially achieved thanks to numerous processes (such as gas-liquid absorption in solvents, adsorption, cryogeny . . . ). Numerous strategies are currently explored in order to identify the most efficient and less expensive process, which could reach a high CO2 capture ratio (typically 80% or more), together with the production of a carbon dioxide stream of high purity (typically a CO2 volume fraction of 0.8 or more). From the energy requirement point of view, the EU has fixed 2 GJ (thermal basis) per ton of carbon dioxide captured as a target. In a first step, gas separation membranes have been discarded for this application, but recent studies suggest that they could possibly offer interesting potentialities.This chater proposes an overview and analyzes the pros and cons of polymeric membrane systems, with an emphasis on post combustion capture in an industrial context (e.g. power plants, steel or cement manufacturing). In a first part, the overall framework of CCS is presented (different sources, capture, transportation, storage) and the key issues of the capture step discussed. The simulation methodology of polymeric gas separation processes is exposed in a second part. Attention is focussed on key variables in process design (membrane selectivity, stage cut, pressure ratio, dimensionless surface area.…) The major objectives of the capture process are quantitatively analysed in the next section: selectivity, energy requirement, capacity. Parametric sensitivity towards purity and carbon dioxide capture ratio is illustrated through some examples. At this stage, two basic strategies can be identified in order to achieve the targets: A single membrane stage approach, which most often requires a high CO2/N2 selectivity material (typically 100 or more),A multistage approach based on membrane materials which show a moderate CO2/N2 selectivity (classically 50 to 70)In the last part, unexplored issues and remaining challenges such as the sensitivity to minor flue gas components (oxygen, water, NOx, SOx…) or novel approaches based on hybrid processes are discussed. The key points raised throughout the above study, and the forthcoming challenges, are listed in the conclusion.